This invention relates to an exhaust gas purification apparatus in an internal combustion engine wherein harmful exhaust gas components such as CO, HC and others are decreased at the catalytic converter, by supplying secondary air to the exhaust passage.
A known exhaust gas purification apparatus for an internal combustion engine includes a means for injecting additional intake air into the intake passage and a means for introducing secondary air into the exhaust passage. The means for injecting additional intake air includes a valve interposed in a passage connected to the intake passage of the engine on the downstream side of the throttle valve. It is arranged to open for a predetermined time by a rapid increase of intake negative pressure and inject additional intake air into the intake passage. The secondary air introducing means includes a control valve interposed in a passage connected to the exhaust passage of the engine on the upstream side of the catalytic converter. It is arranged to open at the time of low load operation of the engine and introduce secondary air into the exhaust passage. The injection of additional intake air prevents the air-fuel ratio of the intake mixture gas from being too rich at the initial period of engine starting, at the initial period of deceleration, at the time of gear changing operation or the like when the intake negative pressure is rapidly increased. Additionally, at the time of low load operation such as idling, deceleration or the like, the harmful ingredients in the exhaust gas are burned in the catalytic converter by the introduction of the secondary air to the exhaust passage. Furthermore, an excess of the secondary air cools the catalytic converter to prevent it from overheating.
In this type of apparatus, the introduction of the secondary air to the exhaust passage is stopped at the time of high load operation of the engine. This is because in the high load operation, the amount and temperature of exhaust gas increases. Therefore, if the secondary air is introduced, overheating damage of the catalytic converter would be likely.
Accordingly, for improving the purification of the exhaust gases at the time of high load operation, it is necessary that the air-fuel ratio of the mixture be set to be lower than the theoretical air-fuel ratio. If, however, the air-fuel ratio is set to become too lean the responsiveness and power of the engine are lowered. In order to keep the engine parameters satisfactory, it becomes necessary that the air-fuel ratio be set on the lean side near the theoretical air-fuel ratio.
If, however, the air-fuel ratio is so set near the theoretical ratio, the harmful components, especially CO, are generated at the time of gear change operation in the midst of the high load operation. It then becomes necessary to remove the same in the catalytic converter by introduction of secondary air. It can be considered, in this case, that the introduction of secondary air can be carried out by utilizing the means for injecting additional intake air which is operated at the time of gear change operation, but sufficient introduction of the air for such a short time cannot be carried out. If, in reverse thereto, the length of time for the introduction of air is prolonged, overheating of the catalytic converter is caused in the high load operation subsequent to the gear change operation.